1. Field of the Invention
The invention relates generally to a process for adjusting the characteristics of infinitely variable transmissions, in particular hydrostatic transmissions, and more particularly transmissions in vehicles having separate travelling drive mechanisms for each side of the vehicle. The invention particularly concerns a process wherein the output speeds of the transmissions are detected and compared with one another via an operating variable which determines the gear ratio, and as a function of a deviation from the operating variable, the gear ratio is adjusted in order to match the characteristics and thus the output speeds of the transmissions.
In the case of vehicles where each side of the vehicle is driven by an infinitely variable transmission, for example loading tractors and bulldozers with two caterpillar tracks each driven by a respective hydrostatic transmission, the manufacture of the two transmissions requires a higher degree of accuracy than in the case of vehicles propelled by only a single such transmission. The reason for this is the desire to obtain transmission characteristics for the two transmissions which are as identical as possible to one another. The characteristic of a transmission is defined as the relationship between the output speed of the transmission and an operating variable. Only when the characteristics of the transmissions are identical is synchronous running of the connected components (e.g., caterpillar tracks) obtained. Then, when the value of the operating variable is the same for each transmission, the same output speed is achieved. Otherwise, although the operating variables for both transmissions may be identical, the output speeds of the transmissions will differ and the speed of one side of the vehicle will differ from that of the other. In this situation, the vehicle will unintentionally move along a curved path, thereby necessitating continuous regulatory correction by the driver. In practice, despite efforts to ensure high production quality and consistency, the transmissions exhibit different characteristics. This makes it necessary, before the vehicle is started up, to adjust the two transmissions in order to achieve operation which is as synchronous as possible. This adjustment must be very accurate to minimize so-called synchronism errors. This is also a prerequisite for the reliable functioning of a superimposed synchronism control device.
2. Prior Art
In a known adjustment procedure for crawler drives with electronic control of the gear ratio, the two output speeds are electronically detected and compared with one another via the parameter of the operating variable. The strength of the electric current which acts upon an electromagnetic control valve to determine the delivery capacity of a hydrostatic variable displacement pump and/or the absorption capacity of a hydrostatic variable displacement motor is usually chosen as the operating variable. If the output speeds of the two hydrostatic transmissions differ when the value of the operating variable is the same, different settings of the operating variable on the respective transmissions are required in order to achieve synchronism. Thus, for example, it is possible to change a spring bias counteracting the electromagnetic force on the control valve of one of the transmissions, so that the rate of the change in the delivery capacity or absorption capacity of the hydrostatic machine can be adjusted. Additionally, the respective control stop means can be adjusted to limit the maximum delivery or absorption capacity. Thus, when the operating variable (in this example the control current to an electromagnetic control valve) is at a predetermined value for both transmissions, their output speeds are matched.
The described process requires a considerable outlay of time and materials. A special diagnosis device is required with the aid of which the time-consuming and complicated adjustment work can be executed. The components to be adjusted must be rendered accessible to the adjustment work, which generally necessitates the removal of other components. Furthermore, the above described adjustment procedure is carried out not only for a single point of the characteristic of a transmission, but for several points to enable the characteristics of both transmissions to be matched with sufficient accuracy. Isolated adjustment of the characteristics at only one point can lead to an impairment of the characteristic symmetry at other points. Despite adjustment at a plurality of points (generally three), the characteristics of the two transmissions still are not identical.